1. Field of the Invention
The present invention relates to an optical amplifier that amplifies an optical signal with a wavelength in a long wavelength region using an erbium-doped fiber.
2. Description of the Related Art
As a known optical amplifier, there is an erbium-doped fiber amplifier (hereinafter, simply referred to as xe2x80x9cEDFAxe2x80x9d) incorporating an erbium-doped fiber (hereinafter, simply referred to as xe2x80x9cEDFxe2x80x9d) having a core doped with an erbium ion ER3+, that is one of rare earth elements, for example. In the EDFA, population inversion is formed by directing pumping light into the EDF. When signal light is incident onto the EDF in a state of the population inversion, stimulated emission causes the signal light to be amplified.
In a case of amplifying a plurality of signal light beams using such an optical amplifier, there is a serious problem of gain deviation with wavelength-dependence, i.e., deviation in powers of the amplified signal light beams depending on the wavelengths even if the signal light beams before being amplified have substantially the same power. Hereinafter, the above-mentioned gain deviation is simply referred to as a gain deviation. To suppress the gain deviation is very important in this art.
Recently, in a wavelength-multiplexed transmission system there is a large demand for further enlarging a band in which the amplification occurs. This raises attention to the development of a long-wavelength-band EDFA (hereinafter, referred to as a L-band EDFA) having a gain band approximately at 1580 nm, that is longer than a 1565 nm-band.
FIG. 1 shows the gain deviation of the L-band EDFA when the signal light beams having substantially the same power but having different wavelengths are amplified.
The gain deviation shown in FIG. 1 is accumulated when a plurality of L-band EDFA described above are connected for multiple amplification of the signal light beams, resulting in degradation of a S/N ratio for some wavelengths. In the worst case, at least one of the signal light beams may be eliminated.
There are also various techniques know conventionally for reducing or compensating the gain deviation of the L-band EDFA. The techniques are related to flattening the gain deviation with respect to the change in the input power. However, a change of the gain deviation with respect to the temperature change that is significantly observed in the L-band EDFA has not been precisely understood yet.
In a case of applying the EDFA to an actually-operated system, it is necessary to compensate the characteristics of the EDFA with respect to both the change in the input power and the temperature change.
The change in the input power is considered as making the compensation of the gain deviation difficult in the L-band EDFA, as in that of a C-band EDFA which amplifies a signal light beam in 1550 nm-band. In addition, the temperature-dependency of the EDF itself is also considered as one factor making the compensation of the gain deviation difficult in the L-band EDFA, although it can be ignored in the C-band EDFA. This is because the L-band EDFA has a longer amplification medium than that of the C-band EDFA.
In general, the gain deviation of the L-band EDFA is not uniform with respect to the change in the input power. That is, the gain deviation of the L-band EDFA is varied depending on the change in the input power. This means that the wavelength-dependency of the multi wavelength output signal is varied.
FIGS. 1-6 show the change of the gain deviation of a specific EDFA when the power of the input signal light beam is changed while other conditions are kept the same. FIGS. 2-6 and FIG. 1 show measurement results in cases of the input power Pin of xe2x88x9212 dBm/ch, xe2x88x9215 dBm/ch, xe2x88x9218 dBm/ch, xe2x88x9221 dBm/ch, xe2x88x9224 dBm/ch and xe2x88x9227 dBm/ch, respectively. As shown in FIGS. 2-6, the gain deviations xcex94G in the above-mentioned cases are 0.43 dB, 2.90 dB, 5.10 dB, 7.10 dB, 8.91 dB and 10.40 dB, respectively.
Moreover, the gain deviation is varied largely with the change in the environmental temperature of the EDF itself, even if the input power is substantially the same. FIGS. 7-10 are diagrams showing a relationship between the temperature change and the change of the gain deviation of the specific EDF for which the measurements shown in FIGS. 1-6 are performed, in a case where pumping light has a specific power. The EDF subjected to the measurements shown in FIGS. 7-10 is adjusted to have the gain deviation xcex94G smaller than 0.8 dB for the respective input powers Pin at the environmental temperature of the EDF of 25xc2x0 C. It is apparent from these measurements that when the environmental temperature of the EDF is changed, the gain deviation xcex94G is also changed largely depending on the temperature change for any input power Pin.
As is apparent from the above, it is necessary to make compensation considering two variable factors, i.e., the change in the input power and the temperature change, in order to compensate the gain deviation of the L-band EDFA.
However, the gain cannot be flattened by a known technique for compensating the gain deviation, if the input power and the temperature are changed.
Moreover, there is no specific technique for compensating the change of the gain deviation of the EDF caused by the temperature-dependence of the L-band EDFA.
For the reasons mentioned above, in order to realize a long-distance optical transmission using the L-band EDFA as a relay, it is required to uniformly amplify the input signal light beams having a plurality of wavelengths but substantially the same power and then output the amplified signal light beams.
In order to solve the above-mentioned problems, an optical fiber amplifier according to the present invention includes an erbium-doped optical fiber to which a multiplexed optical signal into which a plurality of optical signals in a long-wavelength band are multiplexed is supplied and which performs amplification in the long-wavelength band; a forward-pumping light source which supplies forward-pumping light to the erbium-doped fiber; and a backward-pumping light source which supplies backward-pumping light to the erbium-doped optical fiber, the backward-pumping light source having an output power variable based on a control signal, wherein the control signal is output from a control circuit based on an input power of the multiplexed optical signal input to the erbium-doped optical fiber and a temperature of the erbium-doped optical fiber, the control signal changing an output power of the backward-pumping light source so as to cancel deviation of the amplification of the multiplexed optical signal.
Alternatively, the control circuit may output a control signal based on the deviation of the amplified multiplexed optical signal output from the erbium-doped optical fiber, so as to change the output power of the backward-pumping light source to cancel the deviation.